1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, particularly to image forming apparatuses configured to improve stability of an image quality by performing gradation correction processing.
2. Description of the Related Art
In conventional image forming apparatuses for recording monochromatic images and color images, the density of output images may become unstable as time passes due to various factors.
In an electro-photographic type image forming apparatus, for example, processing of laser exposure, latent image formation on a photosensitive member, toner development, toner transfer onto a paper medium, thermal fixing, and the like during electro-photographic processing are easily influenced by the surrounding temperature and humidity of the apparatus and the aging variation of component parts. Thus, the amount of toner that is eventually fixed onto the paper medium varies every time, which may result in unstable output density. Such an unstable condition in the output density arises not only on the electro-photographic type image forming apparatus, but also on the image forming apparatuses of various types such as an ink jet recording type and a thermal transfer type.
As a technique for improving the above-described density variation caused by the environmental factors (such as temperature and humidity), a technique of forming a gradation correction look-up table (LUT) unique to printers has been employed. In order to form the gradation correction LUT, firstly, a test chart on which a gradation pattern is printed is output, and a luminance signal of the gradation pattern printed on the test chart is read by a sensor. Thereafter, the luminance signal is transformed to a density signal through logarithm (LOG) transformation. The density signal acquired therefrom represents a density characteristic of the printer. Therefore, the gradation correction LUT is formed in such a manner that the density characteristic thereof becomes linear. When the above-described operation is performed, a system user (a user or a service person) operates an image forming apparatus according to the instruction from an operation unit, so that the image forming apparatus outputs a test chart. Then, the system user places the test chart on a document positioning plate. Further, by using a sensor that is disposed on a rear portion of a conveyance path from which the paper inside the image forming apparatus is discharged after toner being fixed thereon, the test chart that is output after passing through a fixing unit can be read automatically. This enables the gradation correction to be automatically performed without asking time and effort of the user.
The above-described environmental factors such as temperature and humidity may vary depending on the number of printing pages. For example, the temperature of the fixing unit has a characteristic in that the temperature is high at the time of starting, lowered every time the paper passes therethrough, and gradually stabilized when printing is performed consecutively. In addition, high temperature in the fixing unit results in high-gloss output, so that the printing density tends to be high. In other words, the density of the printing material that is output immediately after the start of printing can easily become high. Therefore, there is a high possibility that an image may not be output in an appropriate density.
In order to perform gradation correction while taking the above-described characteristics into consideration and assuming gradations in a stable condition as essential, above-described, a method may be employed in which a plurality of test charts is printed, and a last-output test chart having a stable gradation characteristic is read as a test chart to form a correction table.
With the above-described method, the gradation characteristic after the gradations have been stabilized can be corrected. However, in the method, a condition in which the gradation characteristic has not been stabilized for a period immediately after printing is actually started, is not taken into consideration. Therefore, output for a period when the gradation characteristic has not been stabilized is executed only by using the correction table whose gradation characteristic is different from the gradation characteristic for that period.
In order to deal with such variations in gradations depending on the number of printing pages from the start of printing, Japanese Patent Application Laid-Open No. 2010-176011 discusses a technique capable of making the output stable from the first printing page by applying the gradation correction according to the number of printing pages.
In the technique, a correction amount suitable for variations in development contrast caused by lowering of an electric potential according to the number of printing pages for a period of consecutive or intermittent printing, is acquired based on environmental sensor information.
Further, in the technique, a gradation correction table is controlled based on the acquired correction amount.
In the above-described technique discussed in Japanese Patent Application Laid-Open No. 2010-176011, the correction amount is always an approximate predictive correction amount based on the environmental sensor information. However, when printing target data is actually printed and output, there is an individual difference in each engine characteristic, and in addition, various types of recording media are used for printing. Accordingly, the correction amount acquired by scanning the gradation pattern, which is output to determine the correction amount with respect to the printing target data, may not be output at high precision.